This invention relates to flexible viscoelastic polyurethane foams used in bedding and furniture cushions. Produced at above atmospheric conditions from certain foaming mixtures, the foams of this invention exhibit a unique combination of low energy losses, high room temperature firmness and high temperature sensitivity.
Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture generally comprised of a polyester or polyether polyol, an isocyanate, a surfactant, catalyst and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure. The surfactant stabilizes the structure.
Once the foam-forming ingredients are mixed together, it is known that the foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, isocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and airtight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
Polyurethane foams with varying density and hardness or firmness may be formed. Hardness or firmness is typically measured as IFD (xe2x80x9cindentation force deflectionxe2x80x9d). Specifically, IFD25 is the force required to compress the foam to 25% of its original thickness or height using the test method set out in ASTM D-3574. Tensile strength, tear strength, compression set, air permeability, fatigue resistance, support factor, and energy absorbing characteristics may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing. Among other things, polyurethane foams are widely used for bedding and furniture cushioning applications.
All or almost all polyurethane foams undergo a transition from a rigid glass-like state to a soft rubber-like state. Over that transition, the foam is viscoelastic. Viscoelastic polyurethane foams are typically characterized by high vibration damping, body conformance and slow recovery from compression. Viscoelastic foams generally have high energy losses. While some consumers prefer a conforming, high-energy loss mattress, an equal number prefer a resilient mattress.
Energy losses may be measured with a dynamic mechanical analyzer, which measures the energy storage modulus and the energy loss modulus of a foam sample under compression at a specified frequency over a range of temperatures. One type of dynamic mechanical analyzer is DMTA IV made by Rheometric Science of Piscataway, New Jersey. Two measurements are taken: Exe2x80x2 is the storage modulus, which indicates the sample""s ability to store energy; Exe2x80x3 is the loss modulus, which indicates the sample""s ability to dissipate energy. From this data, one can measure the ability of the foam sample to store and dissipate the energy. Because it is difficult to use the absolute values of the storage and loss moduli to analyze the mechanical behavior of a foam sample, frequently the ratio of the loss modulus to the storage modulus (Exe2x80x3/Exe2x80x2) is calculated. This ratio is called the tan delta (tan xcex4). Tan delta is the ratio of the energy lost compared to the energy recovered. The higher the tan delta, the higher is the energy loss. For polyurethane foams, the energy loss typically is in the form of heat. For a perfectly elastic polymer, the tan delta is zero. For typical viscoelastic foams with higher energy losses, the tan delta is about 0.6.
Energy losses also may be measured with a ball rebound test (ASTM D 3574) in which a steel ball is dropped from a fixed height onto a foam sample. The ball is allowed to bounce back from the sample surface, and the height the ball reaches on the first rebound is compared to the original drop height. The percent of height in rebound is reported. A higher number indicates a more resilient material. For typical viscoelastic foams with high energy losses, the ball rebound is below about 8%.
Viscoelastic foams have gained popularity for bedding applications because such foams are advertised as reducing pressure points, which are believed to cause tossing and turning during sleep. For example, TEMPUR-PEDIC mattresses and pillows from Tempur-pedic, Inc. of Lexington, Kentucky, are formed of a viscoelastic foam. While many end-users enjoy the body conformance offered by viscoelastic (slow-recovery) foam mattresses, others have complained about having to xe2x80x9cstruggle to climb out of bed,xe2x80x9d which translates to the high energy losses exhibited by viscoelastic foams.
Another unique feature of viscoelastic foams is a strong temperature sensitivity, whereby the foam firmness varies with temperature. For most typical slabstock polyurethane foams, the viscoelastic transition occurs at about xe2x88x9250xc2x0 C., which is termed the glass transition temperature of the foam. Hence, at room temperatures of 20 to 25xc2x0 C., such foams do not have viscoelastic properties and cannot be used for room temperature viscoelastic applications. Some commercially available viscoelastic foams have glass transition temperatures just below about +5xc2x0 C., which still prohibits their use for room temperature viscoelastic applications.
U.S. Pat. No. 5,669,094 (Swanson) discloses a mattress construction having a viscoelastic open celled polyurethane foam as a top layer. The viscoelastic foam is stated to have temperature sensitivity such that its hardness changes between 4xc2x0 C. and 10xc2x0 C. A preferred viscoelastic foam was CONFOR CF-40, which had IFD25 values of 35 lbf at 10xc2x0 C., 4 lbf at 21xc2x0 C. and 3 lbf at 38xc2x0 C. U.S. Pat. No. 6,052,851 (Kohnle) similarly suggests using a CONFOR CF-40 or CF-42 viscoelastic foam as a conforming layer in a mattress construction. Such foams are excessively soft at room temperatures, and do not exhibit significant temperature sensitivity over a temperature range from 70xc2x0 F. to 110xc2x0 F. (21xc2x0 C. to 43xc2x0 C.). Moreover, these patents do not disclose methods for making viscoelastic foams with room temperature sensitivity and high IFD25 at room temperature without significant foam energy losses. See also U.S. Pat. Nos. 6,256,821 (Boyd) and 5,960,496 (Boyd).
U.S. Pat. No. 5,855,415 (Lilley, Jr.) describes a portable seat cushion having an upper foam layer with an impact resilience of 15% or less and with temperature-sensitive compression stiffness response in the temperature range of 10xc2x0 C. to 40xc2x0 C. The foam has an IFD between 10 and 25 and a density of from 24 kg/m3 to 40 kg/m3 (1.5 to 2.5 pcf). The patent does not disclose any method for making such foam.
Given that some consumers may prefer a firm mattress, where others prefer a softer mattress, it would be advantageous if the consumer were able to adjust the firmness of the mattress after purchase by some controllable means, such as varying the temperature. Desirably, a foam for a mattress construction should have an IFD25 range of about 20 to 55 lb within a temperature range of about 70xc2x0 F. to 110xc2x0 F. The prior art does not show mattress and cushion constructions including polyurethane foams that have high firmness at room temperature (about 68xc2x0 F. to 77xc2x0 F. (20xc2x0 C. to 25xc2x0 C.)), coupled with strong temperature sensitivity over this range a low energy losses.
High firmness, low energy loss polyurethane foams with strong temperature sensitivity at or near room temperature are continually sought for bedding and furniture applications. The prior art does not disclose such foams or methods for making them.
According to the invention, flexible, high firmness, temperature sensitive viscoelastic polyurethane foams with low energy losses are produced using a method comprising preparing a foam reaction mixture and foaming that mixture at above atmospheric pressure conditions, preferably at pressures in the range of 1.05 to 1.5 bar (absolute), most preferably 1.1 to 1.3 bar (absolute). The reaction mixture contains: (a) a polyol mixture of (i) about 30 to 95 percent by weight total polyols of a polyether polyol having a hydroxyl number in the range of about 120 to 280 and a functionality from 2.7 to 3.3, and (ii) about 5 to 70 percent by weight total polyols of a graft polyol having a hydroxyl number in the range of about 25 to 50 and a functionality from 2.5 to 3.0; (b) an organic polyisocyanate selected from the group consisting of toluene diisocyanate and toluene diisocyanate mixed with methylene diisocyanate, wherein if a mixture of toluene diisocyanate and methylene diisocyanate is used, the polyisocyanate mixture comprises from about 5 to 20 percent by weight toluene diisocyanate and about 80 to 95 percent by weight methylene diisocyanate, wherein at least 50 percent of the methylene diisocyanate is 4, 4xe2x80x2 methylene diisocyanate, and wherein the isocyanate index is at or below 120, and preferably in the range of 80 to 120, most preferably 80 to 95; and (c) from about 1.5 to 3.0 parts per hundred parts polyol of water as a blowing agent.
Most preferably, the foam-forming composition contains up to 2 parts per hundred parts polyol of an amine catalyst, up to 2 parts per hundred parts polyol of a surfactant, up to 0.5 parts per hundred parts polyol of an organotin catalyst, and up to 2 parts per hundred parts polyol of a cross linking agent.
The resulting viscoelastic polyurethane foams have densities in the range of about 1.5 to 4 pounds per cubic foot, preferably about 2 to 3.5 pounds per cubic foot, and an IFD25 of about 20 to 60 lb at 72xc2x0 F. (22xc2x0 C.) and an IFD25 of about 15 to 35 lb at 100xc2x0 F. (38xc2x0 C.). The foams have a temperature sensitivity such that when the foam is heated from room temperature to about 100xc2x0 F. (38xc2x0 C.), the IFD25 is reduced by 25% or more from the room temperature IFD25 value. The foams further have a ball rebound of at least 9%.
The viscoelastic foams according to the invention may be incorporated into mattresses or mattress pads, seat cushions, automotive seating, cushions or components. When so incorporated, the foam or a portion of such foam may be heated to reduce the firmness, preferably (as measured by IFD25) by at least 25% from its room temperature firmness.